*UPDATED per recommendation*

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Sale 1: (credits to Frosty the Foamman)

https://marketplace.tf/items/518;5;u57;strange (Dec 5th) = $50

Key value (Dec 5th) = $1.7

= 50 / 1.7

= 29.411...

≈ 29 keys

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Any new evidence, help and other suggestions will be greatly appreciated!